The invention pertains to a television receiver whereby an interlaced video signal containing fields is reproduced flicker-free after analog-to-digital conversion, digital processing, and digital-to-analog conversion, comprising digital memories for the digital video signal, a digital movement detector, and a digital field interpolator whose output provides the digital flicker-free video signal.
The picture quality of currently obtainable television receivers is so good that any further improvement appears impossible in view of the inherent properties of the television systems currently in use. The main reason for this lies in the interlaced scanning system, in which each television picture is divided into two fields which are transmitted at a frequency of 50 Hz or 60 Hz. This results in two types of spurious effects, namely in large-area flicker at the field frequency, which is particularly annoying in the European 50-Hz systems, and in so-called edge flicker, which occurs at horizontal edges of objects in the picture at the frame frequency of 25 Hz or 30 Hz. The latter effect not only is very annoying, especially when characters are superposed on the screen, but also reduces the subjective picture resolution, because fine structures appear to dance and cannot be distinguished very well.
To eliminate flicker, the pictures must be presented at a rate higher than the limit of the response of the human eye, which is about 70 Hz. This requires picture memories in the television receiver, and switching must take place between different picture-frequency-increasing methods depending on whether moving or still scanning lines are present, cf. G. Drechsler, "Tagungsband der 11. Jahrestagung der FKTG", June 1984, pp. 558 to 578.
To illustrate one possibility of eliminating flicker, let us consider three successive frames A, B, C, each of which is divided into two fields A1, A2; B1, B2; C1, C2 in interlaced scanning, and which are transmitted and received in this order. One way of avoiding flicker is to present each pair of fields twice, so that frame A is formed on the screen by the four fields in the order A1, A2, A1, A2, which are reproduced at twice the field frequency. This doubling eliminates large-area flicker completely, and edge flicker to a large extent.
However, difficulties are encountered with moving scenes, because, after the later movement phase contained in the second field A2, the earlier phase of the field A1 appears again, which results in the display of jerky and jagged movements that are unpleasant to view. In the presence of movements, therefore, it is necessary to switch to the field sequence A1, A1, A2, A2, so that the fields are doubled in direct succession.
The switching between these two possibilities of reproduction is effected from picture element to picture element by means of the above-mentioned movement detector, which determines the degree of movement on the basis of the difference between the last picture and the newly received picture information. For movement detection, information delayed by one frame must thus be available from the frame memory. Since, in practice, switchover from one field sequence to another results in further considerable picture disturbance, it is known from the above reference to gradually change from one field sequence to the other depending on the degree of movement.
In the reproducing method just described, the field frequency is doubled, as mentioned above. Since secondary effects, such as line crawl, cannot be eliminated in this manner, the above reference indicates that the field pairs can be combined into fields and reproduced sufficiently fast. However, nothing is said in the reference about the number of memories required for this purpose.
German Offenlegungsschrift DE No. 32 03 978 A1 describes a field-frequency-doubling arrangement which uses three field memories. These are interconnected by a suitable arrangement of multipoint switches. The received digital video signal is written alternately into two field memories. As long as the signal is being written into the first field memory at the normal field rate, the preceding field stored in the second field memory is available for readout at twice the write-in rate. With the third field memory, a further signal delay is produced, so that, if the switches are operated in a suitable manner, a pair of successive fields is available at the output of each of the three field memories. Theoretically, both doubly fast field reproduction and corresponding frame reproduction can be achieved with this system.
As investigations have shown, considerable difficulties are encountered in the practical realization of this arrangement. For example, in certain readout phases, the fields must be simultaneously read from and written into the third field memory at double speed, so that this memory must operate at four times the incoming data rate.